1. Field of the Invention
This invention relates to telephone systems and, more particularly, to arrangements for establishing a simultaneous interchange of information among three or more telephone lines.
2. Description of the Prior Art
Ordinarily, the conventional telephone instrument employs a pair of conductors for both the transmitting and the receiving functions, which bilateral transmission arrangement is known in the art as two-wire operation. In ordinary telephone service where communication is established between two telephones, the instruments are simply wired in parallel. Such simple parallel wiring serves adequately for two or three telephones; however, as the number of parallel-wired telephone instruments is increased, the information signal transmitted by any one telephone becomes so severely loaded by the plural parallel receivers that the signal received by each is reduced to an unacceptably low level. Obviously, signal amplification would solve this problem, but the two-wire line is employed for both receiving and transmitting, and available amplifiers are unidirectional.
The prior art has thrust at the above problem by providing arrangements whereby the transmitting and receiving functions are separated from one another in such a manner that each employs a pair of conductors--four-wire operation. Operational amplifiers of hybrid construction are generally employed to accomplish the two-wire to four-wire conversion. A circuit employing operational amplifiers to accomplish the conversion is disclosed in U.S. Pat. No. 3,828,146, which issued on Aug. 6, 1974 to T. G. Lewis.
In some prior art circuits, the two-wire to four-wire conversion circuit is incorporated into the conferencing bridge. One such arrangement which is well known in the art is disclosed in U.S. Pat. No. 3,108,157, which issued on Oct. 22, 1963 to A. Feiner. Feiner discloses a conferencing arrangement employing controlled inductive coupling in addition to operational amplifiers. Severe impedance mismatches are employed to effectively isolate some transformer port pairs, while matched impedances couple other port pairs. Such selective isolation of transformer port pairs prevents regenerative recirculation and enables the use of amplifiers. However, such a predictable inductive coupling arrangement requires close manufacturing tolerances which are maintained at the expense of weight, size, and cost.
A major problem with prior art conferencing arrangements lies in the recirculative nature of hybrid port circuits which connect the two-wire telephone lines to the four-wire PBX. Each hybrid port circuit provides a conductor pair for transmitting signals into the PBX and a conductor pair for receiving signals from the PBX, thereby permitting the use of unidirectional amplifiers. However, signals received by a hybrid port circuit are not completely isolated from the hybrid port circuit's transmitting conductor-pair, with the result that a portion of the received signals is retransmitted via the PBX to the conference circuit. The retransmitted contributions of the plural hybrid port circuits in a PBX which contains a conferencing arrangement can add in amplitude and phase, resulting in a sustained oscillating state which yields objectionable distortion and howling noise, rendering the conference virtually useless. The obvious method of reducing the possibility of this form of instability is to improve the hybrid port circuits to decrease the amount of coupling between the transmit and receive paths and thereby reduce the magnitude of the retransmission. Such coupling is minimized when the input impedance of the hybrid port circuit matches that of the telephone line. The impedance of the telephone line, however, is a function of line length, and, therefore, not predeterminable.
In addition to the above-mentioned hybrid port circuits which connect the two-wire lines to the four-wire PBX, the PBX in which the specific illustrative embodiment of the present invention is used employs a second type of port circuit, which is hereinafter referred to as a conference interface port circuit, for interfacing the four-wire PBX and the subject conference circuit. A conference interface port is provided to transmit to the conference circuit the conference signal from an associated conferee, and to transmit to the conferee via the PBX the conferenced signals received from the conference circuit. However, the operational characteristics of a PBX in which the specific illustrative embodiment of the invention may be employed necessitate that each conference interface port perform a signal subtraction function. The signal received by the conference interface port from the PBX is comprised not only of the conference signal from the associated conferee, but also portions of the already combined conference signals. The combined conference signals are redelivered to the conference interface ports from two sources. First, the hybrid port circuits which connect the two-wire telephone conferee to the four-wire PBX retransmit to the PBX, and consequently to the conference interface port, a portion of the received combined conference signals. Second, the PBX contains summing amplifiers which combine the already combined conference signals onto the bus which carries the TDM signals from the individual conferees. Each conference interface port circuit, therefore, must subtract the combined conference signals from the received signals to recover the signal from the individual conferee. A prior art port circuit which performs a similar signal subtraction function is disclosed in detail in U.S. Pat. No. 3,835,259 which issued on Sept. 10, 1974 to D. G. Medill and P. A. Vachon. It should be noted, however, that the conference interface ports employed to connect the illustrative embodiment of the present invention to the PBX do not subtract all of the combined conference signals from the received signals. Only the combined conference signals which return to the conference interface ports as a result of the signal summing function in the PBX are subtracted. These can be subtracted because the gain of the summing amplifiers in the PBX is known. The magnitude of the combined conference signals which return to the conference interface ports as a result of retransmission by the hybrid port circuits is not so determinable, and, therefore, such combined conference signals are passed on with the conference signal from the individual conferee to the conference circuit where they are cancelled in accordance with the principles of the present invention.
Accordingly, it is an object of this invention to provide an economical conference arrangement.
It is another object of this invention to provide a conference arrangement which remains stable for different numbers of conferees,